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A popular three pin 12 V DC voltageregulator IC.

Voltage regulatorFrom Wikipedia, the free encyclopedia

A voltage regulator is an electrical regulator designed to automaticallymaintain a constant voltage level. A voltage regulator may be a simple"feed-forward" design or may include negative feedback control loops. Itmay use an electromechanical mechanism, or electronic components.Depending on the design, it may be used to regulate one or more AC orDC voltages.

Electronic voltage regulators are found in devices such as computerpower supplies where they stabilize the DC voltages used by theprocessor and other elements. In automobile alternators and centralpower station generator plants, voltage regulators control the output ofthe plant. In an electric power distribution system, voltage regulators may be installed at a substation or alongdistribution lines so that all customers receive steady voltage independent of how much power is drawn from theline.

Contents

1 Measures of regulator quality2 Electronic voltage regulators3 Electromechanical regulators4 Coil-rotation AC voltage regulator5 AC voltage stabilizers

5.1 Electromechanical5.2 Constant-voltage transformer

6 DC voltage stabilizers7 Active regulators

7.1 Linear regulators7.2 Switching regulators7.3 Comparing linear vs. switching regulators7.4 SCR regulators7.5 Combination (hybrid) regulators

8 Example linear regulators8.1 Transistor regulator8.2 Regulator with an operational amplifier

9 See also10 External links11 References

Measures of regulator quality

The output voltage can only be held roughly constant; the regulation is specified by two measurements:

Voltage regulator - Wikipedia, the free encyclopedia http://en.wikipedia.org/wiki/Voltage_regulator

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load regulation is the change in output voltage for a given change in load current (for example:"typically 15mV, maximum 100mV for load currents between 5mA and 1.4A, at some specifiedtemperature and input voltage").line regulation or input regulation is the degree to which output voltage changes with input (supply)voltage changes - as a ratio of output to input change (for example "typically 13mV/V"), or the outputvoltage change over the entire specified input voltage range (for example "plus or minus 2% for inputvoltages between 90V and 260V, 50-60Hz").

Other important parameters are:

Temperature coefficient of the output voltage is the change in output voltage with temperature(perhaps averaged over a given temperature range), while...Initial accuracy of a voltage regulator (or simply "the voltage accuracy") reflects the error in outputvoltage for a fixed regulator without taking into account temperature or aging effects on outputaccuracy.Dropout voltage is the minimum difference between input voltage and output voltage for which theregulator can still supply the specified current. A Low Drop-Out (LDO) regulator is designed to workwell even with an input supply only a Volt or so above the output voltage. The input-outputdifferential at which the voltage regulator will no longer maintain regulation. Further reduction ininput voltage will result in reduced output voltage. This value is dependent on load current andjunction temperature.Absolute maximum ratings are defined for regulator components, specifying the continuous andpeak output currents that may be used (sometimes internally limited), the maximum input voltage,maximum power dissipation at a given temperature, etc.Output noise (thermal white noise) and output dynamic impedance may be specified as graphsversus frequency, while output ripple noise (mains "hum" or switch-mode "hash" noise) may be givenas peak-to-peak or RMS voltages, or in terms of their spectra.Quiescent current in a regulator circuit is the current drawn internally, not available to the load,normally measured as the input current while no load is connected (and hence a source ofinefficiency; some linear regulators are, surprisingly, more efficient at very low current loads thanswitch-mode designs because of this).Transient response is the reaction of a regulator when a (sudden) change of the load current (calledthe load transient) or input voltage (called the line transient) occurs. Some regulators will tend tooscillate or have a slow response time which in some cases might lead to undesired results. This valueis different from the regulation parameters, as that is the stable situation definition. The transientresponse shows the behaviour of the regulator on a change. This data is usually provided in thetechnical documentation of a regulator and is also dependent on output capacitance.

Electronic voltage regulators

A simple voltage regulator can be made from a resistor in series with a diode (or series of diodes). Due to thelogarithmic shape of diode V-I curves, the voltage across the diode changes only slightly due to changes incurrent drawn. When precise voltage control is not important, this design may work fine.

Feedback voltage regulators operate by comparing the actual output voltage to some fixed reference voltage.Any difference is amplified and used to control the regulation element in such a way as to reduce the voltageerror. This forms a negative feedback control loop; increasing the open-loop gain tends to increase regulationaccuracy but reduce stability (avoidance of oscillation, or ringing during step changes). There will also be atrade-off between stability and the speed of the response to changes. If the output voltage is too low (perhapsdue to input voltage reducing or load current increasing), the regulation element is commanded, up to a point, to


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Circuit design for a simple electromechanical voltageregulator.

A voltage stabilizer using electromechanical relaysfor switching.

produce a higher output voltageby dropping less of the input voltage (for linear series regulators and buckswitching regulators), or to draw input current for longer periods (boost-type switching regulators); if the outputvoltage is too high, the regulation element will normally be commanded to produce a lower voltage. However,many regulators have over-current protection, so that they will entirely stop sourcing current (or limit thecurrent in some way) if the output current is too high, and some regulators may also shut down if the inputvoltage is outside a given range (see also: crowbar circuits).

Electromechanical regulators

In electromechanical regulators, voltage regulation is easilyaccomplished by coiling the sensing wire to make anelectromagnet. The magnetic field produced by the currentattracts a moving ferrous core held back under springtension or gravitational pull. As voltage increases, so doesthe current, strengthening the magnetic field produced bythe coil and pulling the core towards the field. The magnet isphysically connected to a mechanical power switch, whichopens as the magnet moves into the field. As voltagedecreases, so does the current, releasing spring tension orthe weight of the core and causing it to retract. This closesthe switch and allows the power to flow once more.

If the mechanical regulator design is sensitive to smallvoltage fluctuations, the motion of the solenoid core can beused to move a selector switch across a range of resistancesor transformer windings to gradually step the output voltageup or down, or to rotate the position of a moving-coil ACregulator.

Early automobile generators and alternators had amechanical voltage regulator using one, two, or three relaysand various resistors to stabilize the generator's output atslightly more than 6 or 12 V, independent of the engine'srpm or the varying load on the vehicle's electrical system.Essentially, the relay(s) employed pulse width modulation toregulate the output of the generator, controlling the fieldcurrent reaching the generator (or alternator) and in this waycontrolling the output voltage produced.

The regulators used for DC generators (but not alternators)also disconnect the generator when it was not producing electricity, thereby preventing the battery fromdischarging back into the generator and attempting to run it as a motor. The rectifier diodes in an alternatorautomatically perform this function so that a specific relay is not required; this appreciably simplified theregulator design.

More modern designs now use solid state technology (transistors) to perform the same function that the relaysperform in electromechanical regulators.

Electromechanical regulators are used for mains voltage stabilisationsee AC voltage stabilizers below.


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Graph of voltage output on a time scale.

Basic design principle and circuit diagramfor the rotating-coil AC voltage regulator.

Magnetic mains regulator

Coil-rotation AC voltage regulator

This is an older type of regulator used in the 1920s that usesthe principle of a fixed-position field coil and a second fieldcoil that can be rotated on an axis in parallel with the fixedcoil.

When the movable coil is positioned perpendicular to thefixed coil, the magnetic forces acting on the movable coilbalance each other out and voltage output is unchanged.Rotating the coil in one direction or the other away from thecenter position will increase or decrease voltage in thesecondary movable coil.

This type of regulator can be automated via a servo controlmechanism to advance the movable coil position in order to providevoltage increase or decrease. A braking mechanism or high ratiogearing is used to hold the rotating coil in place against the powerfulmagnetic forces acting on the moving coil.

AC voltage stabilizers

Electromechanical

Electromechanical regulators, usually called voltage stabilizers,have also been used to regulate the voltage on AC power distributionlines. These regulators operate by using a servomechanism to select theappropriate tap on an autotransformer with multiple taps, or by movingthe wiper on a continuously variable autotransfomer. If the outputvoltage is not in the acceptable range, the servomechanism switchesconnections or moves the wiper to adjust the voltage into the acceptableregion. The controls provide a deadband wherein the controller will notact, preventing the controller from constantly adjusting the voltage("hunting") as it varies by an acceptably small amount.

Constant-voltage transformer

An alternative method is the use of a type of saturating transformercalled a ferroresonant transformer or constant-voltage transformer.These transformers use a tank circuit composed of a high-voltage resonant winding and a capacitor to produce anearly constant average output with a varying input. The ferroresonant approach is attractive due to its lack ofactive components, relying on the square loop saturation characteristics of the tank circuit to absorb variationsin average input voltage. Older designs of ferroresonant transformers had an output with high harmonic content,leading to a distorted output waveform. Modern devices are used to construct a perfect sine wave. Theferroresonant action is a flux limiter rather than a voltage regulator, but with a fixed supply frequency it canmaintain an almost constant average output voltage even as the input voltage varies widely.

The ferroresonant transformers, which are also known as Constant Voltage Transformers (CVTs) or ferros, arealso good surge suppressors, as they provide high isolation and inherent short-circuit protection.


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A ferroresonant transformer can operate with an input voltage range 40% or more of the nominal voltage.

Output power factor remains in the range of 0.96 or higher from half to full load.

Because it regenerates an output voltage waveform, output distortion, which is typically less than 4%, isindependent of any input voltage distortion, including notching.

Efficiency at full load is typically in the range of 89% to 93%. However, at low loads, efficiency can drop below60% and no load losses can be as high as 20%. The current-limiting capability also becomes a handicap when aCVT is used in an application with moderate to high inrush current like motors, transformers or magnets. In thiscase, the CVT has to be sized to accommodate the peak current, thus forcing it to run at low loads and poorefficiency.

Minimum maintenance is required. Transformers and capacitors can be very reliable. Some units have includedredundant capacitors to allow several capacitors to fail between inspections without any noticeable effect on thedevice's performance.

Output voltage varies about 1.2% for every 1% change in supply frequency. For example, a 2-Hz change ingenerator frequency, which is very large, results in an output voltage change of only 4%, which has little effectfor most loads.

It accepts 100% single-phase switch-mode power supply loading without any requirement for derating, includingall neutral components.

Input current distortion remains less than 8% THD even when supplying nonlinear loads with more than 100%current THD.

Drawbacks of CVTs (constant voltage transformers) are their larger size, audible humming sound, and high heatgeneration.

DC voltage stabilizers

Many simple DC power supplies regulate the voltage using a shunt regulator such as a zener diode, avalanchebreakdown diode, or voltage regulator tube. Each of these devices begins conducting at a specified voltage andwill conduct as much current as required to hold its terminal voltage to that specified voltage. The power supplyis designed to only supply a maximum amount of current that is within the safe operating capability of the shuntregulating device (commonly, by using a series resistor).

If the stabilizer must provide more power, the shunt regulator output is only used to provide the standard voltagereference for the electronic device, known as the voltage stabilizer. The voltage stabilizer is the electronicdevice, able to deliver much larger currents on demand.

Active regulators

Active regulators employ at least one active (amplifying) component such as a transistor or operationalamplifier. Shunt regulators are often (but not always) passive and simple, but always inefficient because they(essentially) dump the excess current not needed by the load. When more power must be supplied, moresophisticated circuits are used. In general, these active regulators can be divided into several classes:

Linear series regulators


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Switching regulatorsSCR regulators

Linear regulators

Main article: Linear regulator

Linear regulators are based on devices that operate in their linear region (in contrast, a switching regulator isbased on a device forced to act as an on/off switch). In the past, one or more vacuum tubes were commonlyused as the variable resistance. Modern designs use one or more transistors instead, perhaps within an IntegratedCircuit. Linear designs have the advantage of very "clean" output with little noise introduced into their DCoutput, but are most often much less efficient and unable to step-up or invert the input voltage like switchedsupplies. All linear regulators require a higher input than the output. If the input voltage approaches the desiredoutput voltage, the regulator will "drop out". The input to output voltage differential at which this occurs isknown as the regulator's drop-out voltage.

Entire linear regulators are available as integrated circuits. These chips come in either fixed or adjustable voltagetypes.

Switching regulators

Main article: Switched-mode power supply

Switching regulators rapidly switch a series device on and off. The duty cycle of the switch sets how muchcharge is transferred to the load. This is controlled by a similar feedback mechanism as in a linear regulator.Because the series element is either fully conducting, or switched off, it dissipates almost no power; this is whatgives the switching design its efficiency. Switching regulators are also able to generate output voltages which arehigher than the input, or of opposite polarity something not possible with a linear design.

Like linear regulators, nearly-complete switching regulators are also available as integrated circuits. Unlikelinear regulators, these usually require one external component: an inductor that acts as the energy storageelement. (Large-valued inductors tend to be physically large relative to almost all other kinds of componentry,so they are rarely fabricated within integrated circuits and IC regulators with some exceptions. [1][2])

Comparing linear vs. switching regulators

The two types of regulators have their different advantages:

Linear regulators are best when low output noise (and low RFI radiated noise) is requiredLinear regulators are best when a fast response to input and output disturbances is required.At low levels of power, linear regulators are cheaper and occupy less printed circuit board space.Switching regulators are best when power efficiency is critical (such as in portable computers), exceptlinear regulators are more efficient in a small number of cases (such as a 5V microprocessor often in"sleep" mode fed from a 6V battery, if the complexity of the switching circuit and the junctioncapacitance charging current means a high quiescent current in the switching regulator).Switching regulators are required when the only power supply is a DC voltage, and a higher outputvoltage is required.At high levels of power (above a few watts), switching regulators are cheaper (for example, the costof removing heat generated is less).


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SCR regulators

Regulators powered from AC power circuits can use silicon controlled rectifiers (SCRs) as the series device.Whenever the output voltage is below the desired value, the SCR is triggered, allowing electricity to flow intothe load until the AC mains voltage passes through zero (ending the half cycle). SCR regulators have theadvantages of being both very efficient and very simple, but because they can not terminate an on-going halfcycle of conduction, they are not capable of very accurate voltage regulation in response to rapidly-changingloads. An alternative is the SCR shunt regulator which uses the regulator output as a trigger, both series andshunt designs are noisy, but powerful, as the device has a low on resistance.

Combination (hybrid) regulators

Many power supplies use more than one regulating method in series. For example, the output from a switchingregulator can be further regulated by a linear regulator. The switching regulator accepts a wide range of inputvoltages and efficiently generates a (somewhat noisy) voltage slightly above the ultimately desired output. Thatis followed by a linear regulator that generates exactly the desired voltage and eliminates nearly all the noisegenerated by the switching regulator. Other designs may use an SCR regulator as the "pre-regulator", followedby another type of regulator. An efficient way of creating a variable-voltage, accurate output power supply is tocombine a multi-tapped transformer with an adjustable linear post-regulator.

Example linear regulators

Transistor regulator

In the simplest case emitter follower is used, the base of the regulating transistor is directly connected to thevoltage reference:

The stabilizer uses the power source, having voltage Uin that may vary over time. It delivers the relativelyconstant voltage Uout. The output load RL can also vary over time. For such a device to work properly, the inputvoltage must be larger than the output voltage and Voltage drop must not exceed the limits of the transistor used.

The output voltage of the stabilizer is equal to UZ - UBE where UBE is about 0.7v and depends on the loadcurrent. If the output voltage drops below that limit, this increases the voltage difference between the base andemitter (Ube), opening the transistor and delivering more current. Delivering more current through the sameoutput resistor RL increases the voltage again.


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Regulator with an operational amplifier

The stability of the output voltage can be significantly increased by using an operational amplifier:

In this case, the operational amplifier drives the transistor with more current if the voltage at its inverting inputdrops below the output of the voltage reference at the non-inverting input. Using the voltage divider (R1, R2 andR3) allows choice of the arbitrary output voltage between Uz and Uin.

See also

Constant current regulatorDC to DC converterLinear regulatorVoltage regulator moduleThird brush dynamoPower supply

External links

EMRI - Universal Voltage Regulators (http://www.emri.nl)Designing Regulated Power Supplies (http://knol.google.com/k/max-iskram/electronic-circuits-design-for/1f4zs8p9zgq0e/4)Electrostatic Precipitator Knowledge Base (http://www.neundorfer.com/knowledge_base/electrostatic_precipitators.aspx)

References^ National Semiconductor (http://www.national.com/mpf/LM/LM2825.html#Overview) , http://www.national.com/mpf/LM/LM2825.html#Overview, retrieved 2010-09-19

1.

^ Linear Technology Module Regulators (http://www.linear.com/docs/25633) , http://www.linear.com/docs/25633,retrieved 2011-03-08

2.

Retrieved from "http://en.wikipedia.org/wiki/Voltage_regulator"


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